Raman spectroscopy is an analytical technique used for rapid molecular-level fingerprinting of chemical substances, enabling (for example) real-time detection of contamination/adulteration of food, analysis of paints to determine art forgeries, and immediate identification of white powders that could be controlled substances. However, for any given molecular compound, the Raman spectral frequencies can be used to identify/confirm that compounds are actually fingerprints of specific structural fractions that make up the larger compound; structurally different molecular compounds can show similar fingerprints because they may have specific structural fractions in common. Raman spectroscopy relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range.
The Raman effect occurs when electromagnetic radiation interacts with the polarizable electron density of a given Raman active molecule in the solid, liquid or gas phase. Typically, a sample is illuminated with a laser. Electromagnetic radiation from the illuminated spot is collected with a lens and sent through a monochromator. Elastic scattered radiation at the wavelength corresponding to the laser line (Rayleigh scattering) is filtered out. A small percentage of the laser light is inelastically scattered and interacts with the molecule, resulting in the energy of the laser photons being shifted up or down. This collected light is dispersed onto a detector by either a notch filter or a band pass filter. The shift in energy gives information about the vibrational modes in the system. Infrared spectroscopy yields similar, but complementary, information.
By design, Raman spectra are steady-state measurements. Heat causes more rigid sites within a molecule to increase in flexibility. When collecting Raman spectral measurements during an applied thermal gradient, some sites remain equally rigid during heating while other sites change flexibility in response to the temperature gradient. Variable temperature thermodynamic-based Raman spectroscopy (VTR) measurement techniques and apparatus described herein are unique because the temperature gradient enables distinguishing of vibrational modes that are due to kinetic processes from those that are due to steady-state processes; VTR identifies the precise temperature and/or temperature range over which molecular changes occur, and, concomitantly, the specific molecular sites most directly involved in these changes. The VTR method and apparatus described herein enables differentiation of more elastic sites (relatively temperature dependent vibrations) from more rigid sites (relatively temperature independent vibrations).
Although multiple spectroscopic techniques are available that detect molecular level changes in mobility, no instrumentation/methodology prior to the VTR process (described herein) reports and provides evidence for identifying molecular level sites of elasticity. The need exists for a Raman spectroscopic method and apparatus that can quickly and reliably distinguish similar compounds and provide molecular structural information for a target substance. The method and apparatus described herein provide a protocol and apparatus to enable a user to reliably identify compounds that are not readily distinguishable using conventional Raman spectroscopy as well as provide molecular structural information regarding a target substance.